Along with the barrel, the bolt carrier is perhaps the most essential component of an AR-15 rifle. The bolt carrier houses the bolt and literally carries and guides it within the receiver. It also resets the hammer after firing. There are several types of bolt carriers available for use with the AR-15, as well as a number of different bolt carrier coatings.

AR-15 Bolt Carriers

Along with the M-16, the AR-15 is one of two main types of bolt carriers found in the AR-15 rifle. The AR-15 bolt carrier a.k.a. semi-auto bolt carrier has a larger cutout at the bottom than the M16 styled BCG. This results in a reduced weight, which is preferable to some AR-15 users. Also, there is no difference in the bolt used with either the AR-15 or M-16 bolt carrier. The AR-15 is generally the less expensive of the two major bolt carrier versions.

M-16 Bolt Carriers

The M-16 bolt carrier a.k.a. full-auto bolt carrier features a shorter, full diameter section at its back end, which increases the overall weight of the BCG by about an ounce. The additional weight adds to the stability of the bolt carrier. The M-16 is also generally required for users who want fully automatic firing capabilities for their rifles. The M-16 bolt carrier is thought to slow down the cycle rate, which is also preferable for fully automatic rifles.

Mil-Spec Bolt Carriers

The term “mil-spec” means that a product is designed and built to adhere to the standards set forth by the U.S. military. In order for a bolt carrier to be considered mil-spec, it must meet the following requirements:

Chrome plating: The interior of a mil-spec bolt carrier must be chrome plated. Plating is a metal finishing process where a coating of a specific metal is electroplated onto the surface of another metal object. Chrome plating helps to prevent corrosion and also increases the bolt carrier’s durability.

Shot peening: A mil-spec bolt carrier is subjected to shot peening, which is a metal finishing process where small glass, ceramic or metallic materials called shot are projected onto the surface of a metal object at high intensity. Shot peening causes changes in the mechanical properties of the surface of the object, significantly increasing metal fatigue life.

Grade 8 fasteners: Grade 8 fasteners consist of carbon-alloy steel that is heat-treated in a carbon-controlled environment. They are regarded as the toughest and most durable of all the fastener types.

Half-Circle Bolt Carriers

While most AR-15 bolt carriers feature an end with a full-circle design, there are also half-circle bolt carriers available. With half-circle bolt carriers, a portion of the cylindrical end is milled off to create a semi-circular design. The area beneath and behind the firing pin, referred to as the “shroud,” has also been milled off. This makes for an even lighter bolt carrier than the standard AR-15 version.

Low Mass / Lightweight Bolt Carrier

In addition to the bolt carrier types mentioned above, low-mass or lightweight bolt carriers have also become very popular in 3-gun competition circles. As is implied by the name, switching from a standard BCG to a low-mass BCG lowers the overall mass of rifle components that move when the rifle is fired, which reduces felt recoil as well as the movement of the rifle while firing rapid shots in succession. In addition to this, cycling is faster and smoother, which is one of the main reasons this type of BCG is preferred in 3-gun environments.

One important thing to keep in mind about low-mass bolt carriers is that, like most things, an advantage in one area can mean a disadvantage in another. While a lower mass does increase the speed and reduce the weight of the rifle, low-mass bolt carriers are also more susceptible to wear and tear, and as a result are typically not suited to be chosen as a lifetime component for an AR-15 setup.

AR-15 Bolt Carrier Coatings

There are a number of coatings available for AR-15 bolt carriers. A metal coating offers a number of important benefits, particularly in terms of enhancing corrosion protection and durability. Your choice of the best coating for your AR-15 bolt carrier depends on factors such as the type of environment in which you use your weapon, the quality of the bolt carrier and how much money you are willing to spend. Common AR-15 bolt carrier coatings include nickel boron and phosphate.

Nickel boron bolt carrier: A nickel boron coating is applied to a bolt carrier via a process known as electroless plating. Unlike electroplating, electroless plating does not depend on electricity to deposit the nickel boron onto the surface of the bolt carrier. Instead, deposition occurs by an autocatalytic chemical reaction.

Nickel boron offers resistance against corrosion. It also increases lubricity and makes the bolt carrier easier to clean. In addition to this, it also has a desirable silvery metallic appearance. Nickel boron can also be buffed to achieve a gold chrome finish, although this tends to produce glare that would be a disadvantage when concealment is necessary.

Phosphate bolt carrier: Phosphating, also known as Parkerizing, entails the application of a manganese phosphate coating to the surface of the bolt carrier. With phosphating, the bolt carrier is submerged in a phosphoric acid solution (which contains the manganese) and is heated to a temperature of 190–210 °F.

A manganese phosphate coating will increase the thickness of the bolt carrier. Phosphating tends to provide greater durability than a nickel boron coating, while also increasing wear resistance. It is also possible to alter the color of the coating from medium-gray to black.

Other Considerations When Selecting a Bolt Carrier

Testing is an important factor when choosing a bolt carrier for your AR-15. To ensure you’re getting a high-quality bolt carrier, check to see that it has been both magnetic particle inspected (MPI) and high-pressure tested (HPT), as these will verify the product is free of cracks and other issues that could impact performance and longevity.

Also conduct some research to determine whether the manufacturer individually tests each bolt carrier or conducts batch testing (where they test only one product in every completed batch). Most reputable bolt carrier manufacturers conduct individual testing.

You will also want to check that you are getting a properly staked bolt carrier in terms of the gas key. The gas key is the bolt carrier component that comes in contact with the gas tube extending from the gas block on the barrel. You will want to be sure the bolts that attach the gas key are “staked” in place to prevent them from loosening under pressure during firing.

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